1. Field of the Invention
The present invention relates to an organic light-emitting device to be used in, for example, a flat panel display, a projection display, or a printer.
2. Description of the Related Art
Organic light-emitting elements (organic EL elements or organic electroluminescence elements) have been actively researched and developed these days. It has been known that an aluminum-lithium alloy or a magnesium-silver alloy is used in the cathode of any such organic light-emitting element in order that electron injection efficiency may be improved. It has been also known that an extremely thin layer (having a thickness of 5 to 10 Å) of a dielectric substance such as lithium, lithium fluoride, magnesium oxide, or potassium fluoride is inserted into an electron injection layer in contact with the cathode.
Japanese Patent Application Laid-Open No. H10-270171 discloses a constitution in which an electron injection layer (having a thickness of 10 to 3,000 Å) having a metal capable of functioning as a donor (electron-donating) dopant is provided in contact with a cathode. Examples of a disclosed metal to be used as the donor dopant include alkali metals, alkali earth metals, and transition metals including rare earth elements.
Japanese Patent Application Laid-Open No. H10-270172 discloses a constitution in which an electron injection layer (having a thickness of 10 to 2,000 Å) having a metal oxide or a metal salt as a dopant is provided in contact with a cathode.
Meanwhile, efforts have been made to improve the light extraction efficiency of an organic light-emitting element. Japanese Patent Application Laid-Open No. 2000-323277 discloses a constitution in which an organic compound layer contacting with from an organic light-emitting layer to a metal electrode is formed so as to have such a thickness that an optical distance from a light-emitting interface to an interface between the organic compound layer and the metal electrode is substantially equal to an odd multiple of ¼ of the wavelength of emitted light. Such constitution maximizes light interference between the emitted light and returned light by reflection on the metal electrode, thereby improving light extraction efficiency.
Japanese Patent Application Laid-Open No. 2004-132189 discloses a constitution in which an organic compound layer is formed so as to have such a thickness that an optical distance between an anode and a cathode is equal to an integral multiple of ½ of the wavelength of emitted light. Such constitution provides a resonator structure in which light beams reflected between the anode and the cathode intensify each other, thereby improving light extraction efficiency.
In a conventional constitution using a single layer of an electron injection material such as lithium fluoride (LiF) or potassium fluoride (KF) as an electron injection layer, a thickness capable of electron injection is extremely small, specifically, 5 to 10 Å, so that it has been difficult to improve light extraction efficiency by controlling the thickness of the electron injection layer. On the other hand, in the case of an electron injection layer obtained by doping an organic compound with an electron injection material, a thickness capable of electron injection is 50 to 3,000 Å.
However, extensive studies made by the inventors of the present invention have shown that, even in the case of an electron injection layer obtained by doping an organic compound with an electron injection material, electron injection efficiency reduces with decreasing thickness of the layer, with the result that the voltage at which a light-emitting element is driven increases. Further, a reduction in emission efficiency has occurred as a result of the loss of the optimum carrier balance in some cases. For example, there arises the following problem: when the thickness of the electron injection layer of an organic light-emitting element capable of emitting blue light having a short wavelength is reduced for improving the light extraction efficiency of the element, an increase in driving voltage of the element and a reduction in emission efficiency occurs.
The present invention provides an organic light-emitting device including a plurality of organic light-emitting elements different in luminescent color from each other, in which light extraction efficiency for the luminescent color of each organic light-emitting element can be improved without an increase in driving voltage of the element and a reduction in emission efficiency.